Question

I want to create a TreeMap in Java with a custom sort order. The sorted keys which are string need to be sorted according to the second character. The values are also string.

Sample map:

Za,FOO
Ab,Bar

Answer 1

You can use a custom comparator like this:

    Comparator<String> secondCharComparator = new Comparator<String>() {
        @Override public int compare(String s1, String s2) {
            return s1.substring(1, 2).compareTo(s2.substring(1, 2));
        }           
    };

Sample:

    SortedMap<String,String> map =
        new TreeMap<String,String>(secondCharComparator);
    map.put("Za", "FOO");
    map.put("Ab", "BAR");
    map.put("00", "ZERO");
    System.out.println(map); // prints "{00=ZERO, Za=FOO, Ab=BAR}"

Note that this simply assumes that the String has a character at index 1. It throws StringIndexOutOfBoundsException if it doesn’t.

---

Alternatively, you can also use this comparison:

return s1.charAt(1) - s2.charAt(1);

This subtraction “trick” is broken in general, but it works fine here because the subtraction of two char will not overflow an int.

The substring andcompareTo solution above is more readable, though.

See also:

• Java Integer: what is faster comparison or subtraction?